In recent years, pushbutton switches employed in information equipment have been required to have a deeper stroke in order to obtain better tactile key operation, and to have a lower profile so that portable machines can be made thinner. In this market situation, how to realize a lower profile pushbutton switch that has a longer key-top, such as a space bar, than other ordinary key-tops is discussed.
A conventional pushbutton switch that has a longer key-top is described hereinafter with reference to the accompanying drawings. FIG. 10 is a front view, half in cross section, of the conventional pushbutton switch. FIG. 11 is a lateral cross section taken on line 11--11 of FIG. 10.
In FIG. 10, switch member 200 having switch contact 210 sandwiched by two films, on which conductive paste is printed, is disposed on metallic substrate 100. Elastic rubber dome 300 includes a protrusion 310, which depresses switch contact 210, at an upper inside portion thereof. Cylindrical stem 410 is provided underneath and at the center of key-top 400. Stem 410 pushes against outer wall 320, which is donut-shaped as viewed from above, near a top of the rubber dome 300, and is pushed upwardly by elasticity of rubber dome 300.
Two sets of resin-molded link members 500, each set comprising two members 500 crossing each other at their center and fixed at the crossing point and forming an "X" shape, are disposed so that the two sets sandwich the rubber dome 300. Resin-molded fixture 600 is placed on an upper face of switch member 200. An upper end 510 of link member 500 is engaged rotatably with stopper 420 provided on a lower face of key-top 400, while a lower end 520 is rotatably and slidably engaged in recess 610 of fixture 600. These engagements are symmetrically constructed with regard to rubber dome 300, the same engagements discussed above are thus provided on the left half of FIG. 10.
Cylindrical depending bosses 430 are provided on both sides on the lower face of key-top 400, and an outer wall of each boss 430 can slide with regard to an inner wall of a corresponding guide hole part 620 protruding from the fixture 600. The hole 620 is oval in plan view.
FIG. 12 shows a perspective view of a stabilizing member 700 formed by bending a hard metal wire. Stabilizing member 700 has a rectangular shape with open at the center.
Another side 710 of the stabilizing member 700 is engaged rotatably with two stoppers 440 disposed on the lower face of the key-top 400 near both ends of this side 710. The open side opposite the to side 710 is held slidably between an upper face of fixture 600 and two hook-shaped protrusions 630 at terminal portions 720. These engagements of stabilizing member 700 are symmetrically arranged with regard to rubber dome 300.
An operation of the conventional pushbutton switch having the construction discussed above is described hereinafter.
When a user depresses key-top 400 with a fingertip, the upper end 510 of link member 500, which is engaged with the stopper 420, is rotated. At the same time, the lower end 520 held in the recess 610 of fixture 600 is rotated and slid. Stem 410 thus depresses the outer wall 320 of rubber dome 300, whereby, rubber dome 300 is deflected, and then the protrusion 310 activates switch contact 210.
When the user releases the pressure from key-top 400, the rubber dome 300 returns to its natural orientation, and moves the link member 500 and the key-top 400 to their upper positions. Switch contact 210 then returns to the off-status shown in FIG. 10.
In this operation, the outer circumference of boss 430 shown in FIG. 11 slidably contacts the inner wall of guide hole 620 across the shorter diameter of its oval shape, and slides up and down with regard to the inner wall in response to the up and down travelling of key-top 400.
Regarding the stabilizing member 700, when the side 710 held by stopper 440 is rotated, both terminals 720 of the side opposite to the side 710 slide in a horizontal direction between protrusions 630 and the upper face of fixture 600.
In other words, the looseness of key-top 400 in a rotating direction with regard to rubber dome 300 and in a horizontal direction is prevented by the engagement of the cylindrical boss 430 in the oval guide hole 620. The looseness in a vertical direction on both sides of key-top 400 is prevented by stabilizing member 700.
The conventional pushbutton switch having a longer key-top, however, cannot be free from looseness or imbalance of heights on both sides of key-top 400, even with all the preventive measures discussed above, because of gaps in holders of both terminals 720 and warping of key-top 400.
In order to prevent boss 430 from coming out of guide hole 620, some height for engagement is desirably prepared between the boss 430 and the hole 620, which prevents heights of both of the boss and guide hole from being further lowered. As a result, the conventional pushbutton switch is limited to its original height.
Regarding a deeper stroke of the pushbutton switch, it requires greater heights of boss 430 and guide hole 620.